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An international team of scientists working with the Event Horizon Telescope project have been able to measure the radius of a black hole for the first time.

It was in 1781 that astronomer Charles Messier first observed the galaxy M87 - without knowing, at the time, that it was definitely a galaxy. 231 years later, scientists observing M87 have just done something that's never been done before - they've measured the radius of the supermassive black hole at its center. A supermassive black hole that's 50 million light years away and is 6 billion times more massive than the Sun.

It wasn't easy, though. The radius of the black hole was observed with the Event Horizon Telescope - a chain of radio telescopes in Hawaii, Arizona, and California. By combining these telescopic observations, scientists are able to obtained unprecedented resolutions when observing objects in the night sky. In this case, what the astronomers were able to observe the matter at the edge of the black hole as that matter approached the event horizon.

The event horizon of a black hole is "an exit door from our universe," explained one of the astronomers in an MIT press release. "You walk through that door, you’re not coming back."

The scientists were able to determine the radius by examining hot jets of matter that are being ejected from the black hole at near-light speeds. Those ejections are caused by the superheating the black hole's accretion disk - the matter surrounding the black hole - as it approaches the event horizon of the black hole. The rotation of the black hole and the matter around it create powerful magnetic fields, which periodically cause the black hole to violently eject these jets of superheated matter across the Galaxy.

The jets being ejected from M87's black hole have actually been observed before - the Hubble Space Telescope caught a picture of the jets escaping in 1998. But the resolution of Hubble isn't great enough to allow scientists to make the precise measurements they needed to determine the size of the black hole. With the Event Horizon Telescope, astronomers can. The accretion disks innermost orbit is about 70 billion miles - roughly 5 times the size our entire solar system. After careful calculation and observation, they determined that the accretion disk is about 5.5 times the size of the black hole itself, giving them an estimate of the radius.

Measuring the black hole, though, is only the beginning of the research into M87's supermassive black hole. The Event Horizon Telescope will be adding radio telescopes from still more countries into its array, allowing even more precise observation of the black hole. This will allow scientists to verify theories about black hole behavior. It will also shed new insight into Einstein's Theory of Relativity, as astronomers will be able to determine if Einstein correctly predicted what happens to matter in a strong gravitational field.

"We are now in a position to ask the question, 'Is Einstein right?'" said astronomer Shep Doeleman in the release. "We can identify features and signatures predicted by his theories, in this very strong gravitational field."